Molecular dynamics simulation study of nonconcatenated ring polymers in a melt. II. Dynamics

Molecular dynamics simulations were conducted to investigate the dynamic properties of melts of nonconcatenated ring polymers and compared to melts of linear polymers. The longest rings were composed of N = 1600 monomers per chain which corresponds to roughly 57 entanglement lengths for comparable linear polymers. The ring melts were found to diffuse faster than their linear counterparts, with both architectures approximately obeying a D ～ N(-2.4) scaling law for large N. The mean-square displacement of the center-of-mass of the rings follows a sub-diffusive behavior for times and distances beyond the ring extension [linear span]R(g)(2)[linear span], neither compatible with the Rouse nor the reptation model. The rings relax stress much faster than linear polymers, and the zero-shear viscosity was found to vary as η(0) ～ N(1.4 ± 0.2) which is much weaker than the N(3.4) behavior of linear chains, not matching any commonly known model for polymer dynamics when compared to the observed mean-square displacements. These findings are discussed in view of the conformational properties of the rings presented in the preceding paper [J. D. Halverson, W. Lee, G. S. Grest, A. Y. Grosberg, and K. Kremer, J. Chem. Phys. 134, 204904 (2011)].     

Re-entrant phase behavior for systems with competition between phase separation and self-assembly

In patchy particle systems where there is a competition between the self-assembly of finite clusters and liquid-vapor phase separation, re-entrant phase behavior can be observed, with the system passing from a monomeric vapor phase to a region of liquid-vapor phase coexistence and then to a vapor phase of clusters as the temperature is decreased at constant density. Here, we present a classical statistical mechanical approach to the determination of the complete phase diagram of such a system. We model the system as a van der Waals fluid, but one where the monomers can assemble into monodisperse clusters that have no attractive interactions with any of the other species. The resulting phase diagrams show a clear region of re-entrance. However, for the most physically reasonable parameter values of the model, this behavior is restricted to a certain range of density, with phase separation still persisting at high densities.     

Synthesis and characterization of thorium(IV) sulfates

Three Th(IV) sulfates, two new and one previously reported, have been synthesized from aqueous solution. In all of the compounds, the sulfate anions coordinate the Th(4+) metal center(s) in a monodentate manner with Th-S distances of 3.7-3.8 ?. Th(SO(4))(2)(H(2)O)(7)·2(H(2)O) (1; P2(1)/m, a = 7.224(1) ?, b = 12.151(1) ?, c = 7.989(1) ?, ss =98.289(2)°) and Th(4)(SO(4))(7)(H(2)O)(7)(OH)(2)·H(2)O (2; Pnma, a = 18.139(2) ?, b = 11.173(1) ?, c = 14.391(2) ?) each contain 9-coordinate monomeric (1,2) and dimeric (2) Th(IV) cations in monocapped square antiprism geometry. Alternatively, Th(OH)(2)SO(4) (3; Pnma, a = 11.684(1) ?, b = 6.047(1) ?, c = 7.047(1) ?) is built from chains of hydroxo-bridged, 8-coordinate Th(4+) centers. Whereas 1 adopts a molecular structure, 2 and 3 both exhibit 3D architectures. Differences in the dimensionality and the topology of 1-3 are manifested in the local coordination environment about the Th(IV) centers, the formation of oligomeric Th(4+) species, and the extended connectivity of the sulfate ligands. Herein, we report the syntheses and characterization of 1-3 as well as the atomic correlations of 1 in solution, as determined by high-energy X-ray scattering (HEXS).     

A trans-AB-bacteriochlorin building block

Synthetic bacteriochlorins are of interest for fundamental studies in photochemistry because of their strong absorption in the near-infrared spectral region and close similarity with natural bacteriochlorophylls. A de novo route to 5-methoxybacteriochlorins entails self-condensation of a dihydrodipyrrin-acetal, which in turn is prepared from a 2-(2-nitroethyl)pyrrole species and an α,β-unsaturated ketone-acetal (e.g., 1,1-dimethoxy-4-methylpent-3-en-2-one). Here, four new results are reported concerning the synthesis of substituted bacteriochlorins. First, a new, scalable route to 1,1-dimethoxy-4-methylpent-3-en-2-one removes a significant previous impediment to the overall route. Second, the new route was employed to gain access to new α,β-unsaturated ketones and corresponding dihydrodipyrrins bearing alternative substituents in place of the dimethoxy unit. Third, a dihydrodipyrrin bearing a 1,3-dioxolan-2-yl moiety afforded the bacteriochlorin (30% yield) containing a 2-hydroxyethoxy substituent at the 5-position. Fourth, subsequent bromination proceeded regioselectively at the 15-position to give a trans-(5,15)-AB-bacteriochlorin building block. The linear 5,15-substitution pattern is attractive for a number of molecular designs. The results taken together afford deeper understanding of the scope and limitations of the de novo route and also advance the capabilities for tailoring synthetic bacteriochlorins.     

Synthesis and study of redox-active acyclic triazenes: toward electrochromic applications

Coupling of various 4-substituted phenyl azides with two distinct quinone-containing N-heterocyclic carbenes (NHCs) afforded the respective mono- and ditopic 1,3-disubstituted acyclic triazenes in moderate to excellent yields (38-92%). Depending on their pendant substituents (derived from the azides), the acyclic triazenes exhibited intense absorptions in the visible spectrum (359-428 nm), which were bathochromically shifted by up to Δλ=68 nm upon reduction of the quinone moiety on the component derived from the NHC. Cyclic voltammetry confirmed that the aforementioned redox processes were reversible, and a related set of UV-vis spectroelectrochemical experiments revealed that bulk electrolysis may also be used to switch reversibly the colors exhibited by these triazenes.     

Synthesis-guided structure revision of the sarcodonin, sarcoviolin, and hydnellin natural product family

A sweeping structural revision of the sarcodonin natural product family (published structures 1a-13a) is proposed after extensive studies aimed at their chemical synthesis. Key features of revised structure 1b include replacement of the N,N-dioxide moiety with an oxime, ring-opening of the central diketopiperazine, and transposition of the terphenyl wing from the 1β-2β position of 1a to the 2β-3β position of 1b. This structure revision arose from the serendipitous synthesis of a benzodioxane aminal (44) whose structure was unambiguously determined by X-ray crystallography and whose spectral properties bore considerable resemblance to the published data for the sarcodonins. A versatile new method for O-arylation of hydroxamic acids is also reported herein, as well as a manganese(III)-mediated α-oxidation of hydroxamic acids to aminals.     

Relation between two-photon absorption and dipolar properties in a series of fluorenyl-based chromophores with electron donating or electron withdrawing substituents

We investigate two-photon absorption (2PA) in a series of fluorenyl-based 9,9-diethyl-2-ethynyl-7-((4-R-phenyl)ethynyl)-9,9a-dihydro-4aH-fluorene chromophores with R being various electron donating (ED) and electron withdrawing (EW) groups. We use wavelength-tunable femtosecond laser pulses to measure the 2PA cross sections in the lowest dipole-allowed transition and show that the substituents with stronger ED or EW character enhance the peak 2PA cross section (up to σ(2) ～ 60-80 GM) while the neutral substituents lead to smaller cross sections, σ(2) < 10 GM. We apply two-level approximation to establish a quantitative relation between the 2PA in the pure electronic transition (0-0) and the corresponding change of the permanent electric dipole moment upon the excitation (Δμ). This relation is elucidated by comparing Δμ values obtained from the 2PA measurements with quantum-chemical calculations and with measurements of solvatochromic shifts in a series of solvents. We show that the calculated Δμ correlate well with the values obtained from the 2PA spectroscopy. The Δμ values obtained from the solvatochromic shifts agree well with the above two methods for the chromophores with neutral or weak EW or ED substituents. On the other hand, stronger EW or ED end groups give much larger Stokes shifts, which lead to an overestimation of the Δμ values. We tentatively attribute this effect to the excitation-induced electronic density change occurring predominantly at the substituent side of the molecule, which causes the effective point dipole associated with the Δμ to interact more strongly with the surrounding solvent.     

Vibrational spectroscopy of N-methyliminodiacetic acid (MIDA)-protected boronate ester: examination of the B-N dative bond

N-methyliminodiacetic acid (MIDA)-protected boronate esters are a new class of reagents that offer great promise in iterative Suzuki-Miyaura cross-coupling reactions. Compared to earlier reagents, MIDA esters are easily handled and are benchtop stable under air indefinitely. The success of this new species is tied to its unique molecular architecture. Compared to the simpler B-N containing molecules ammonia borane and trimethylamine borane, MIDA esters are much larger, and the sp(3) hybridized boron atom is secured by two five membered rings, making this molecular class stable for spectroscopic study. Here, we present infrared, Raman, and surface enhanced Raman (SERS) spectra of methylboronic acid MIDA ester. Comparisons of the spectroscopic results to those from electronic structure calculations suggest that the B-N stretching mode in this molecule lies in the range 560-650 cm(-1), making it among the lowest energy vibrations observed to date that can be primarily attributed to B-N stretching.     

Tridentate ionic hydrogen-bonding interactions of the 5-fluorocytosine cationic dimer and other 5-fluorocytosine analogues characterized by IRMPD spectroscopy and electronic structure calculations

Ionic hydrogen-bonding interactions have been found in several clusters formed by 5-fluorocytosine (5-FC). The chloride and trimethylammonium cluster ions, along with the cationic (proton-bound) dimer have been characterized by infrared multiple-photon dissociation (IRMPD) spectroscopy and electronic structure calculations performed at the B2PLYP/aug-cc-pVTZ//B3LYP/6-311+G(d,p) level of theory. IRMPD action spectra, in combination with calculated spectra and relative energetics, indicate that it is most probable that predominantly a single isomer exists in each experiment. For the 5-FC-trimethylammonium cluster specifically, the calculated spectrum of the lowest-energy isomer convincingly matches the experimental spectrum. Interestingly, the cationic dimer of 5-FC was found to have a single energetically relevant isomer (Cationic-IV) involving a tridentate ionic hydrogen-bonding interaction. The three sites of intermolecular ionic hydrogen bonds in this isomer interact very efficiently, leading to a significant calculated binding energy of 180 kJ/mol. The magnitude of the calculated binding energy for this species, in combination with the strong correlation between the simulated and IRMPD spectra, suggests that a tridentate-proton-bound dimer was observed predominantly in the experiments. Comparison of the calculated relative Gibbs free energies (298 K) for this species and several of the other isomers considered also supports the likelihood of the dominant protonated dimer existing as Cationic-IV.